1. Technical Field
The present invention relates generally to reciprocating electromagnetic devices, and more particularly to a construction of a reciprocating electromagnetic device for reducing secondary losses in magnetically-permeable elements.
2. Related Art
Direct conversion of AC electric power into reciprocating mechanical power by resonant motors (and the reverse conversion in alternators) has become important in applications like pulse-tube and Stirling-cycle cryocoolers and small externally-heated engine-generators operating on a thermoacoustic or Stirling cycle. As with rotary motors and alternators, these devices typically include a moving field (permanent or electromagnet) interacting with a fixed field, typically electrically-conductive windings around magnetically-permeable structure. Alternatively, both fields may be fixed in space, but variably coupled by a moving magnetically-permeable material bridge (such machines are often called ‘variable-reluctance’ devices).
Unlike more common rotary motors, the moving parts in such devices reciprocate, typically along the central axis of the assembly. Such reciprocation causes three-dimensional changes in the magnetic flux fields directed through and around the magnetically-permeable structure (typically iron-based). Magnetic permeability is always associated with electrical conductivity in continuous (homogeneous) materials. Such conductivity gives rise to losses called eddy currents, by acting as closed, conductive circuits around changing magnetic flux vectors.
In rotary practice, the ‘iron’ (so called even when the element iron is not the primary constituent) is built up from layered laminations of thin sheet material to form the intended shape, where each lamination is in a plane perpendicular to the rotation axis. Generally, except near the ends, rotary motors are cylindrical and generally axisymmetric, so that there is no axial gradient of magnetic potential to drive magnetic flux out of a transverse plane like that occupied by the iron laminates. Very little eddy current is generated in such conditions, where field vectors are confined to lie in laminar planes separated by non-conductive coatings.
In many reciprocating machines, for example U.S. Pat. No. 4,602,174 to Redlich or U.S. Pat. No. 4,349,757 to Bhate, an iron path is constructed from axial (radial) or pseudo-radial laminates. Although this approach generally aligns the laminate orientation to the magnetic flux vectors, it is costly and requires special manufacturing processes and equipment not shared with more common rotary machines. Such axially-laminated structures are also not well adapted to mechanical load-bearing, attachment to other structures, or thermal stability in operation.
Another approach to reciprocating devices is taught by U.S. Pat. No. 5,146,123 to Yarr et al. and later adopted in a variation by Nasar in U.S. Pat. No. 5,654,596. In these devices, the standard axially-stacked or layered laminations typical of common rotary motors are used to minimize manufacturing cost. In Yarr et al., special provision is made in shaping the pole tip regions to reduce magnetic flux intensity there and thereby minimize the adverse effects of magnetic flux vectors perpendicular to the electrically-conductive laminations there. No such provisions are made in Nasar, which implies either a higher loss or a lower magnetic flux density throughout, and associated larger mass and cost. In both cases, the magnetic flux is largely confined to laminate planes in most of the device, far from the active magnetic interface between moving and stationary parts. Unfortunately, the losses in the pole area can dominate the total loss inventory, especially at higher frequencies.
Some recent work (e.g., U.S. Pat. Nos. 5,198,137, 5,306,524 and 5,543,174) has been directed towards developing composite materials comprising small, closely-packed particles of conductive, magnetically-permeable material, electrically insulated by a matrix of organic binders. At present, these insulated composites are very costly and such materials exhibit lower total permeability and magnetic saturation levels. This implies larger structures with longer coil pathways that raise the resistive loss in the electrical circuits. Still, they can be molded to shapes that cannot be laminated easily, and they carry magnetic flux in any direction without high eddy current losses because there are no large-scale conductive pathways in the material. As a result, they have been applied as replacements for laminates in reciprocating electromagnetic dynamic devices where the cost, size, and resistance penalties were acceptable, as for instance, in low-power, specialty motors or high-frequency inductors and chokes.
In view of the foregoing, there is a need in the art for an electromagnetic device that provides many of the performance benefits of the more costly axial laminations or insulated composite iron in a reciprocating machine, but with the economical construction of a layered laminate for compatibility with rotary manufacturing practice.